Printing plate registration using a camera

ABSTRACT

A method and apparatus for determining the alignment of printing plate ( 14 ) mounted on an imaging drum ( 12 ), and applying an image to a printing plate while maintaining registration and alignment between the image and a reference edge ( 15 ) of the printing plate include mounting the printing plate on a plate-making machine ( 10 ) and then determining the locations of at least two points on the reference edge. The locations of two or more points are used to determine a transformation that is applied to image data to yield transformed image data ( 27 ). The transformed image data is used to image the printing plate. A method and apparatus for determining the locations of the points using backlighting avoids errors encountered in illuminating from the top.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned, copending U.S. patentapplication Ser. No. 11/204,223, filed Aug. 16, 2005, entitled PRINTINGPLATE REGISTRATION AND IMAGING, by Neufeld et al., the disclosure ofwhich is incorporated herein.

FIELD OF THE INVENTION

The invention relates to printing and, in particular to providingregistered images on printing plates.

BACKGROUND OF THE INVENTION

Printing plates may be imaged on a plate-making machine and thentransferred to a printing press. Once on the printing press, the imagesfrom the printing plates are transferred to paper or other suitablesubstrates. It is important that images printed using a printing pressbe properly aligned with the substrate on which they are printed.Obtaining such alignment typically involves:

-   -   carefully aligning a reference edge of a printing plate with        pins or other features on the plate making machine;    -   detecting one reference point on an orthogonal edge of the        printing plate (i.e. orthogonal to the reference edge) at a        known distance from the reference pins;    -   imaging the printing plate; and    -   using the reference edge and the orthogonal edge reference point        to align the printing plate on a drum of the printing press.

One common technique of aligning the printing plate on the drum of aprinting press involves using the reference edge and the orthogonal edgereference point to align the printing plate on a punching machine andpunching registration holes in the printing plate. The printing platemay then be aligned on the drum of the printing press with registrationpins that project through the registration holes.

Traditionally mechanical alignment pins have been used to align theplate to be imaged to the drum of a platesetter. This is not a flexiblearrangement. The pins have to be mounted in predetermined positions.There are also reliability challenges in consistently and accuratelyloading the plate into contact with the pins. It is also difficult todefine sets of pins that allow a wide range of plate formats to beimaged whilst not interfering with one another.

There is therefore a need for a alignment mechanism not based onmechanical locating pins. A number of these have been proposed.

Techniques for detecting an edge of a printing plate, and any associatedskew in the plate are disclosed in U.S. Pat. No. 6,321,651 (Tice et al.)and in U.S. Pat. No. 6,318,262 (Wolber et al.). In these patents edgedetection sensors are employed in the load path to an imaging drum uponwhich a printing plate is imaged. U.S. Pat. No. 4,881,086 (Misawa)describes a laser recorder with sheet edge detection. The edge detectionworks on the principle of the difference in reflectivity between that ofthe sheet and that of the drum on which it is carried.

European Patent Application EP 1 081 458 A2 (Elior et al.) describes anapparatus for detecting a plate edge including an optical printhead forilluminating a plate system with a light beam having a focal pointgenerally at the plate, a detector for measuring the intensity of thelight reflected from the plate system, and means for altering therelative position of the edge of the plate and the light beam. EP 1 081458 A2 also teach the use of this apparatus to determine a skew angle ofthe plate mounted on a plate support surface.

U.S. Pat. No. 6,815,702 (Kiermeier et al.) describes detecting the edgeof a printing plate mounted on an external drum by using a light sourceand light sensor to measure the difference in reflectivity between theplate and drum surface. Two grooves formed in the drum surface are usedto increase the difference in reflectivity between the plate and thedrum. A difference in the positions where the plate covers each of thegrooves is determined and compared against predetermined value todetermine if a skewed plate is mounted on the drum.

U.S. Pat. No. 4,876,456 (Isono et al.) describe using photosensorshaving light emitting elements and light receiving elements disposed ina path for carrying a photosensitive film. The light emitting elementsemit periodic pulsed light. When the film is present at the locations ofthe photosensors, the periodic pulsed light is reflected by the film toenter the light receiving elements thereby detecting the presence of thefilm. The activation time and activation interval of the light emittingelements are determined so that accumulated exposure value in eachportion on the film is less than an upper limit value of accumulatedexposure value of the photosensitive film.

In commonly-assigned U.S. patent application Ser. No. 11/204,223 an edgedetection system is described, based on using a CCD camera to image theedges of a printing plate perpendicular to the sub-scan direction. Basedon the information so obtained, the image data is then adjusted tocompensate for any misalignment between the plate and the drum on whichit is loaded.

The problem with front side illumination of printing plates on aplatesetter is that the bevelled nature of the aluminium plate edgemakes the determination of the outer mechanical limit of that edgerather difficult. This happens because both the flat top surface of theplate and the bevelled edge are likely to reflect the illuminating lightwhen a CCD camera is used to image the edge.

A further limitation as regards front side illumination arises in theuse of light sensitive plates, in which case light that could expose theemulsion has to be avoided.

There is therefore a need in the printing industry for a method andapparatus capable of consistently and automatically determining theouter mechanical edge of the plate that is to be imaged.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention a method ofimparting a print image on a printing plate comprises:

-   -   a) mounting the printing plate on an imaging drum in an        orientation wherein a reference edge of the printing plate        extends along the drum in a substantially axial direction;    -   b) determining a location of at least one point on the reference        edge, wherein the determining comprises for each of the at least        one point:        -   i) illuminating a region, the region comprising at least a            part of the reference edge associated with each point;        -   ii) capturing at least one digital camera image of the            region; and        -   iii) locating the at least a part of the reference edge in            the digital camera image; and    -   c) imparting the print image on the printing plate in alignment        with at least the determined location of the at least one point.

The present invention provides a method and apparatus for determining analignment of a printing plate and imaging the printing plate inaccordance with the determined alignment. The present inventioncomprises mounting the printing plate on an imaging support surface of aplate making machine and subsequently determining locations of at leastone point on a reference edge of the printing plate relative to thesurface. Based on the determined locations of each point on thereference edge, the method involves adjusting print image data such thata print image corresponding to a print image data is imparted on theprinting plate in alignment with at least the determined locations ofeach point. An illumination source may be used to illuminate a regionassociated with each point, the region including at least a part of thereference edge. A digital camera may be used to take digital cameraimages of each region. The location of each point is determined at leastin part by locating at least a part of the reference edge in eachdigital camera image. When at least one point includes at least twoaxially spaced apart points, a transformation required to impart theprint image on the printing plate in a desired registration relative tothe reference edge is determined. The transformation may include arotation. The method also involves applying the transformation todigital image data to yield transformed image data and using thetransformed image data to image the printing plate.

The present invention makes it unnecessary to accurately align theprinting plate on the plate-making machine prior to imaging the printingplate.

A first aspect of the present invention provides a method of imparting aprint image on a printing plate. The method includes mounting theprinting plate on an imaging drum in an orientation in which a referenceedge of the printing plate extends along the drum in a substantiallyaxial direction. A location of each of the at least one point on thereference edge is determined. Determining the location of each pointincludes illuminating a region, the region including at least a part ofthe reference edge associated with each of the points. Determining eachlocation also includes capturing at least one digital camera image ofeach region, and locating at least a part of the reference edge in thedigital camera image. The method includes imparting the print image onthe printing plate in alignment with at least the determined location ofat least one point.

The at least one point may include at least two axially spaced apartpoints on the reference edge. The method may further includetransforming print image data representing the print image in accordancewith the determined locations of the at least two axially spaced apartpoints, to yield transformed print image data. The print image may beimparted on the printing plate using the transformed print image data,the imparted print image being aligned with the reference edge.

An illumination source may be used for illuminating each region and adigital camera may be used to capture the digital camera images. Theillumination source and the digital camera are moved in an axialdirection relative to the imaging drum. The method of the presentinvention is executed using a continuous light source. The imaging drumis kept stationary during the execution of the method of the invention.Determining the location of the at least one point includes determiningan axial position of the digital camera relative to the drum when thedigital camera is axially movable.

Locating the at least part of the reference edge in the digital cameraimage may include applying a correlation algorithm to distinguish animage of the at least a part of the reference edge from a prototypeedge. Applying a correlation algorithm may include applying a Haartransform. Applying a correlation algorithm may also include applying acoiflet transform. Locating the at least a part of the reference edge inthe digital camera image may include performing a line detectionalgorithm. The printing plate may be substantially rectangular in shape,and include a pair of longer edges and a pair of shorter edges. Thereference edge may be one of the longer edges.

In another aspect of the present invention, a method is provided fordetermining an alignment of a printing plate relative to an imaging drumon which the printing plate is mounted. The method includes determininga location of at least one point on a reference edge of the printingplate. The reference edge extends along the imaging drum in asubstantially axial direction. Determining the location of each pointincludes illuminating a region including at least a part of thereference edge associated with each point. Determining the location ofeach point includes capturing at least one digital camera image of theregion with a digital camera and locating at least a part of thereference edge in the digital camera image. The method provides fordetermining the alignment of the printing plate at least in part fromthe location of at least a part of the reference edge in the digitalcamera image of each point, and a position of the digital camerarelative to the imaging drum during the capturing of at least onedigital camera image of each point.

The at least one point may include at least two axially spaced apartpoints on the reference edge. The imaging drum may include at least oneregistration pin in contact with an edge of the printing plate. Thealignment of the printing plate may further be determined from aposition of at least one registration pin. The digital camera is mountedon a carriage operable for moving in an axial direction relative to theimaging drum. The alignment of the printing plate may further bedetermined from an axial position of the carriage.

In a further aspect of the present invention, a method for determiningthe location of a reference edge of a plate relative to a substrate onwhich the plate is mounted, comprises determining a location of at leastone point on the reference edge, wherein the reference edge extendsalong a first direction on the substrate; the determining a locationcomprising illuminating a region, the region comprising at least a partof the reference edge associated with the at least one point; capturingat least one digital camera image of the region with a digital camera;locating the at least a part of the edge in the digital camera image;and determining the location of the at least one point from (i) a partof the reference edge in the digital camera image of the at least onepoint; and (ii) a position of the digital camera relative to thesubstrate during the capturing of the at least one digital camera imageof the at least one point.

The locating at least a part of the reference edge can comprise locatinga mechanical edge of the printing plate. The illuminating can beperformed on the side of the plate that is in contact with thesubstrate. The illuminating can be spatially interrupted along aninterrupting section of a part of the reference edge associated witheach point. The illuminating can be by diffuse reflected light reflectedfrom at least one diffusely reflective area on the substrate. Theilluminating can be spatially interrupted by at least one non-reflectivesurface on the substrate. The reference edge can be positioned to extendover a slot in the substrate, the slot extending in substantially thefirst direction; and the at least one non-reflective surface and the atleast one diffusely reflective surface can be on a recessed surface ofthe slot. The at least one diffusely reflective area can form an acutereflective apex with the perpendicular projection of the reference edgeonto the recessed surface of the slot. The plate can be held to thesubstrate by at least one clamp and the imaging of the reference edgedone through an imaging aperture in the at least one clamp while theilluminating is done through an illumination baffle in the at least oneclamp. The substrate can be an imaging drum. The at least one point cancomprise at least two spaced apart points on the reference edge. Thedigital camera can be mounted on a carriage operable for moving in thefirst direction relative to the surface and the location of thereference edge determined from a position of the carriage.

This aspect of the method of the invention can be combined with themethod for determining an alignment of a printing plate relative to animaging drum on which the printing plate is mounted and with the methodof imparting a print image on a printing plate.

Yet another aspect of the present invention provides an apparatus forimparting a print image on a printing plate. The apparatus includes asubstantially cylindrical imaging drum. The imaging drum includes meansfor securing the printing plate in an orientation in which a referenceedge of the printing plate extends along the imaging drum in asubstantially axial direction. The apparatus includes an illuminationsource operable for illuminating a region associated with each of atleast one point on the reference edge. Each of the regions includes atleast a part of the reference edge. The apparatus includes a digitalcamera operable for capturing a digital camera image of each region. Theapparatus also includes one or more processors operable for determininga location of each point by at least locating the at least a part of thereference edge in each digital camera image. The one or more processorsare further operable for adjusting print image data representing theprint image in accordance with the determined locations of each point,the adjusted print image data including data to align the print imagewith at least the determined location of each point. The apparatus alsoincludes an imaging head operable for receiving the adjusted print imagedata from the one or more processors and for imparting the print imageonto the printing plate in accordance with the adjusted print imagedata.

The at least one point may include at least two axially spaced apartpoints on the reference edge. The one or more processors may be furtheroperable for transforming the print image data into transformed printimage data based on the determined locations of each of at least twoaxially spaced apart points. The transformed print image data includesdata to align the print image with the reference edge. The image headmay be further operable for imparting the print image on the printingplate in accordance with the transformed print image data, the impartedprint image being aligned with the reference edge.

The apparatus may include a movable carriage operable for moving theimaging head in an axial direction relative to the imaging drum. Thecarriage may be further operable for moving at least one of digitalcamera and the illumination source in the axial direction. The digitalcamera may include one of a CCD sensor and a CMOS sensor. Theillumination source may be a LED. The apparatus may further include acontroller operable for analyzing the image obtained by the digitalcamera.

In yet a further aspect of the present invention, an apparatus fordetermining the location of a reference edge of a plate relative to asubstrate on which the plate is mounted, is operable for illuminatingthe side of the plate that is in contact with the substrate. Thesubstrate can be an imaging drum. The apparatus comprises a slot in thesubstrate, an illumination source operable for illuminating a regionassociated with a point on the reference edge, the region comprising atleast a part of the edge; a digital camera operable for capturing adigital camera image of the region; and one or more processors operablefor determining a location of the point by at least locating the atleast a part of the reference edge in the digital camera image. The slothas a recessed surface and extending along the substrate in a firstdirection. The slot is configured to allow placement of the plate withthe reference edge extending along the substrate in substantially thefirst direction with the reference edge protruding over the slot. Therecessed surface of the slot can comprise at least one diffuselyreflective surface. The recessed surface can comprise at least onesubstantially non-reflective surface. The at least one diffuselyreflective surface can form an acute reflective apex with aperpendicular projection of the reference edge onto the recessedsurface. The apparatus can further comprise at least one clamp forclamping the plate to the substrate at the point. The illuminationsource can be operable for illuminating the reference edge through theat least one clamp while the digital camera can be operable for imagingthe reference edge through the at least one clamp. The at least oneclamp can comprise an illumination baffle and an imaging aperture andthe illumination source can operable for illuminating the reference edgethrough the illumination baffle in the at least one clamp while thedigital camera can be operable for imaging the reference edge throughthe imaging aperture in the at least one clamp. The digital camera canbe operable for imaging the reference edge through the imaging apertureusing light from the illumination source, the light illuminating therecessed surface of the slot through the illumination baffle. Thedigital camera can comprise one of a CCD sensor and a CMOS sensor andthe illumination source can be an LED.

This aspect of the apparatus of the present invention can be employed inan apparatus for determining an alignment of a printing plate relativeto an imaging drum on which the printing plate is mounted and may alsobe employed in an apparatus for imparting a print image on a printingplate, as described here.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following detailed descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which illustrate non-limiting embodiments of theinvention:

FIG. 1 is a schematic diagram of a prior art external drum-typeplate-making machine;

FIG. 2A is an isometric depiction of a printing plate mounted to a drumin the plate-making machine of FIG. 1;

FIG. 2B is a top elevation view of an imaged printing plate in a priorart punching machine;

FIG. 2C is an isometric view of an imaged printing plate mounted on thedrum of a prior art printing press;

FIG. 3 is flow chart illustrating one embodiment of a method for imaginga printing plate according to the invention;

FIG. 4A is an isometric depiction of a printing plate mounted to a drumof a plate-making machine according to a particular embodiment of theinvention;

FIG. 4B shows a plan view of an imaged printing plate mounted in askewed orientation;

FIG. 5 is a schematic illustration of a plate-making machine accordingto one embodiment of the invention;

FIG. 6 is a schematic illustration of a digital camera based arrangementfor imaging the edge of a printing plate by the method of the presentinvention;

FIG. 7 is a cutaway drawing of the drum of a plate-making machine,showing the slot in the drum and the placement of diffusely reflectingand non-reflecting members; and

FIG. 8 is a plan view of the slot in the drum of a plate making machine.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements have not been shown or described indetail to avoid unnecessarily obscuring the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative, ratherthan a restrictive, sense.

Prior Art

FIG. 1 is a schematic depiction of a plate-making machine 10 having adrum 12 on which a printing plate 14 may be mounted. Plate-makingmachine 10 has an imaging head 16 which can impart an image ontoprinting plate 14. In the illustrated embodiment, imaging head 16 isaxially movable relative to drum 12 (i.e. along the directions parallelto the axis of drum 12 indicated by double-headed arrow 24). Imaginghead 16 typically includes a radiation source (not shown), such as alaser, which emits one or more beams of laser radiation capable ofimparting an image onto printing plate 14. A controller 20 controlsimaging head 16 and its associated radiation source in accordance withprint image data stored in a memory 22, so as to image printing plate14. The Trendsetter™ plate-setters available from Eastman Kodak Companyrepresent examples of plate making machines having the basicconfiguration shown in FIG. 1.

FIG. 2A shows drum 12 of plate-making machine 10 in greater detail. Drum12 has a plurality of registration pins 18A, 18B, 18C which project fromits cylindrical surface 13. In the illustrated embodiment, drum 12comprises three registration pins 18A, 18B, 18C, which may be offsetslightly from one another around the circumference of drum 12 to enableimaging of different sizes of printing plates. A reference edge 15 ofprinting plate 14 is brought into engagement with two registration pins18A, 18B. Typically, printing plate 14 is rectangular in shape andreference edge 15 may be one of the long edges of printing plate 14 (asdepicted in FIG. 2A). The shorter, orthogonal edge 19 of printing plate14 extends around the circumference of drum 12. An edge detector (notshown) detects the position of a third reference point 11 on orthogonaledge 19 of printing plate 14. Orthogonal edge reference point 11 islocated at a fixed circumferential distance 23 relative to registrationpins 18. Printing plate 14 is clamped onto drum 12 using any suitableclamping system (not shown). Typically, clamping systems clamp toreference edge 15 and to an opposing edge of printing plate 14 (notshown) that is parallel to reference edge 15.

With printing plate 14 clamped and registered, drum 12 is rotated aboutits axis in either or both of the main-scan directions indicated byarrow 26, while imaging head 16 is scanned axially along drum 12 (i.e.in the directions indicated by arrow 24). Controller 20 controls therelative movement of imaging head 16 and drum 12 and controls theradiation source in imaging head 16 in accordance with print image data27 to impart an print image 17 onto printing plate 14. An edge 17A ofprint image 17 is created substantially parallel to reference edge 15.The region 25 of printing plate 14 that is adjacent to reference edge 15and the region (not shown) that is adjacent to the opposing edge ofprinting plate 14 are covered by the clamping system and are not imaged.

After being imaged on plate-making machine 10, printing plate 14 may bepunched in a punching machine 50 as shown in FIG. 2B. Printing plate 14is registered on punch table 52 of punching machine 50 by bringing itinto engagement with two registration surfaces 18A′, 18B′ on itsreference edge 15 and registration surface 11′ on its orthogonal edge19. The position and orientation of the two registration surfaces 18A′,18B′ on punch table 52 (as measured with respect to each other and withrespect to printing plate 14) may be substantially the same as theregistration pins 18A and 18B on plate-making machine 10. Punch table 52comprises a third registration surface 11′ that is located acircumferential distance 23 from registration pins 18A, 18B, 18C. Thus,registration surface 11′ is located in the same position as orthogonaledge reference point 11 on plate-making machine 10 (see FIG. 2A). Withprinting plate 14 registered to surfaces 18A′, 18B′, 11′, punchingmachine 50 creates a number of punched features (not shown) in printingplate 14. The punched features created by punching machine 50 may have awide variety of shapes, sizes and orientations. However, because theregistration points are the same when printing plate 14 is imaged (18A,18B, 11) and when it is punched (18A′, 18B′, 11′), the locations of thepunched features are known precisely with respect to image 17.

Once printing plate 14 is punched, reference edge 15 and the opposingedge (i.e. parallel to reference edge 15) of printing plate 14 may bebent (not shown). As shown in FIG. 2C, printing plate 14 is then mountedon a press cylinder 62 of a printing press 60. The clamping system (notshown) of printing press 60, which is used to mount printing plate 14 topress cylinder 62, may comprise registration pins (not shown) whichproject through the features punched in printing plate 14 to secureprinting plate 14 to press cylinder 62. The clamping system may also usethe bent edges of printing plate 14 (if present) to secure printingplate 14 to press cylinder 62. When printing plate 14 is securelymounted to press cylinder 62, the clamping system overlaps non-imagedregion 25 of printing plate 14 (i.e. adjacent to reference edge 15) andthe non-imaged region adjacent the opposing edge of printing plate 14(i.e. the edge parallel to reference edge 15). In this manner, theclamping system of printing press 60 does not impede print image 17 onprinting plate 14. Print image 17 is then transferred to a substrate(not shown) by applying ink to printing plate 14 and rolling drum 62 tobring inked image 17 into contact with the substrate.

The Present Invention

FIG. 3 shows a method 100 for registering and imparting print image 17of FIG. 4A onto printing plate 14. FIG. 4A depicts printing plate 14 oncylindrical surface 13 of drum 12 of a plate-making machine according toone embodiment of the present invention. Method 100 begins with block102, which involves mounting printing plate 14 on drum 12 of aplate-making machine. The plate-making machine could be an externaldrum-type plate-making machine, as shown in FIG. 1. Alternatively, theplate-making machine could be an internal drum-type plate-makingmachine.

In the illustrated embodiment of the present invention (FIG. 4A),printing plate 14 is mounted to cylindrical surface 13 of a drum 12 ofan external drum-type plate making machine with its shorter edge 19′extending, at least partially, in a circumferential or main-scandirection 26 around drum 12. The longer reference edge 15′ of printingplate 14 extends in the axial or sub-scan direction along drum 12,clamped by at least two clamps 120 and 130. To assist in positioningprinting plate 14, printing plate 14 may optionally touch at least oneof optional reference pins 18A, 18B, 18C on drum 12 to provide a singleorthogonal edge reference point 21.

At least two clamps 120 and 130, described in more detail below, holdprinting plate 14 on drum 12 of the plate making machine and arethemselves fastened to drum slot 140 in a manner that leaves at least aportion of reference edge 15′ exposed through each of at least twoclamps 120 and 130 in a manner described in more detail below. At leastone clamp 120 and 130 hold printing plate 14 on drum 12 at least tworeference points 28A and 28B. Clamps 120 and 130 may be individualclamps or may be segments of a single larger clamp. The single largeclamp may extend along the whole length, or substantially the wholelength, of drum 12. Clamps 120 and 130 are located in fixed knownpositions.

In block 104 of FIG. 3, the positions of the at least two referencepoints 28A, 28B on reference edge 15′ are determined. Reference points28A, 28B may be found using the backlit edge method and apparatus of thepresent invention, described in detail below.

FIG. 4B shows a plan view of an imaged printing plate 14 that has beenmounted in a skewed orientation with respect to an axis of imaging drum12. If the skew is not addressed, print image 17 may be imparted ontoprinting plate 14 such that an edge 17A of print image 17 may form anangle θ with respect to reference edge 15′. The amount of skewrepresented by angle θ has been exaggerated in FIG. 4B for the purposesof clarity and may be less or more than the angle shown. Referring backto FIG. 3, in block 106 the locations of the at least two referencepoints reference 28A and 28B are used to determine angle θ by whichprint image 17 should be rotated to properly align an edge 17A of printimage 17 with reference edge 15′ of printing plate 14. In block 108, therotation angle θ determined in block 106 is used to generate atransformation to be applied to print image data. The transformation maycombine rotation and translation to map each image point in the printimage data to a transformed image point.

The transformation is applied to print image data in block 110 of FIG. 3to produce transformed image data. The transformation may be determined(in block 108) and applied to print image data (in block 110) by a dataprocessor at the plate-making machine. For example, a processor incontroller 20 may determine the transformation from data provided by theedge finder and apply the transformation to print image data.

In block 112 of FIG. 3, the transformed print image data is used bycontroller 20 to drive imaging head 16 and its associated radiationsource, so that print image 17 is imparted on printing plate 14. Asdiscussed above, imaging head 16 moves in the axial sub-scan directions(see arrow 24 of FIG. 1) to impart print image 17 onto printing plate 14while drum 12 rotates in a main-scan directions (see arrow 26 of FIG.1). To the extent that the axial edges of printing plate 14 need to beknown, they can be determined by the method of commonly-assigned U.S.patent application Ser. No. 11/204,223.

Print image 17 imparted onto printing plate 14 will have an edge 17Athat is aligned with reference edge 15′ of printing plate 14. In someembodiments, print image 17 imparted onto printing plate 14 may havesome other desired registration relative to reference edge 15′. Thisother desired registration may be repeated for other associated platesmade in the plate-making machine to assure registration among all theassociated plates when mounted on a printing press.

After printing plate 14 is imaged, it may be punched on a punchingmachine 50 (see FIG. 2B). Registration surfaces 18A′, 18B′ of punchingmachine 50 are typically aligned with edge detected reference points28A, 28B on reference edge 15′ of printing plate 14. Similarly,registration surface 11′ of punching machine 50 is typically alignedwith orthogonal edge reference point 21. Because of the registration ofreference points 21, 28A, 28B to registration surfaces 11′, 18A′, 18B′,when printing plate 14 is punched, the locations of the punched featuresare known precisely with respect to print image 17. If required,reference edge 15′ and the opposing edge (i.e. parallel to referenceedge 15′) of printing plate 14 may be bent (not shown).

Printing plate 14 may then be mounted onto press cylinder 62 of aprinting press 60 (see FIG. 2C). When printing plate 14 is mounted onthe press cylinder of a printing press, it is preferably mounted suchthat its reference edge 15′ extends in an axial direction (i.e. parallelto the axis of the press cylinder). The clamping system of printingpress 60 may comprise registration pins which project through one ormore of the punched features in printing plate 14 to secure printingplate 14 to press cylinder 62. The clamping system of printing press 60may also use the bent edges of printing plate 14 to secure it to presscylinder 62. Once mounted to press cylinder 62, print image 17 can beinked and transferred to paper and/or other suitable substrates.

In the embodiment of FIG. 5, a digital camera 40 is affixed to carriage101 of an external drum plate-making machine or platesetter. Digitalcamera 40 can include one of a CCD sensor and a CMOS sensor. Carriage101 moves along lead screw 103 in a direction given by arrow 24. Digitalcamera 40 has a known position and orientation relative to drum 12 ofFIG. 4A. In FIG. 5 carriage 101 is shown in a position relative to drum12 that allows the accurate determination of point 28B in particular.Illumination source 105 is affixed to digital camera 40 and illuminatesreference edge 15′ through channels in at least two clamps 120 and 130and digital camera 40 captures images of reference edge 15′ throughopenings in at least two clamps 120 and 130 located at reference points28A, 28B on reference edge 15′ using the backlit edge method andapparatus of the present invention, described in detail below.Illumination source 105 can be an LED. In FIG. 5, illumination source105 is shown in a position to illuminate point 28B through clamp 130 inparticular. The images are processed to identify reference edge 15′ andto accurately determine the locations of the two or more referencepoints 28A, 28B on reference edge 15′. A line detection algorithm may beused to locate reference edge 15′ at each of the at least two referencepoints 28A and 28B. A straight line may be fitted to the locatedreference edge 15′. The positions of the two or more reference points28A, 28B on reference edge 15′ may be determined from the fitted line.Alternatively, the plate-making machine could be an internal drum-typeplate-making machine.

In the illustrated embodiment of the present invention shown in FIG. 5,printing plate 14 is mounted to a drum 12 of an external drum-type platemaking machine with its reference edge 15′ (i.e. the longer edge ofprinting plate 14 in this embodiment) extending axially along thecylindrical surface 13 of drum 12. Reference edge 15′ of printing plate14 may optionally touch at least one of reference pins 18A, 18B, 18C ondrum 12 (see FIG. 4A) to assist in positioning printing plate 14 suchthat its reference edge 15′ protrudes partially over drum slot 140.

At least two clamps 120 and 130 hold printing plate 14 on drum 12 of theplate making machine in a manner that allows illumination source 105 toilluminate reference edge 15′ through channels or illumination bafflesin at least two clamps 120 and 130 and digital camera 40 to captureimages of reference edge 15′ through openings in at least two clamps 120and 130 located at reference points 28A, 28B on reference edge 15′ usingthe backlit edge method and apparatus of the present invention,described in detail below.

In the embodiment shown in FIG. 5, a plate-making machine includes animaging head 16 that is affixed to movable carriage 101. Theplate-making machine also includes mutually affixed illumination source105 and digital camera 40. In other embodiments of the presentinvention, illumination source 105 and/or digital camera 40 may beaffixed to imaging head 16. In other embodiments, digital camera 40,illumination source 105 and imaging head 16 may be variously affixed toone another, or not, or may travel along direction 24 independent of oneanother. A digital camera with a relatively small field of view may beemployed. A digital camera 40 that may employ a small field of viewincludes the Black and White Ultra-Miniature Camera, Model WDH-2500,manufactured by the Weldex Corporation. In this embodiment of thepresent invention, digital camera 40 can be moved over a larger sub-scandistance than the field of view of digital camera 40 to find variouspoints along reference edge 15′ where printing plate 14 is clamped by atleast two clamps 120 and 130. Illumination source 105 and digital camera40 may be employed to capture images of the at least two referencepoints 28A, 28B on reference edge 15′. In other embodiments of thepresent invention, illumination source 105 and digital camera 40 may beemployed to capture digital camera images of various points along thereference edge 15′ of each of a plurality of plates 14 mounted on drum12. In other embodiments of the present invention, illumination source105 illuminates a region that includes at least a part of reference edge15′ associated with least one point found on reference edge 15′. The atleast one point may correspond to one or more of the at least two ormore reference points 28A and 28B.

In yet other embodiments of the present invention, the plate-makingmachine may include a plurality of imaging heads 16. Each of theplurality of imaging heads 16 may be used to image at least one of aplurality of printing plates 14 mounted on drum 12. A separate digitalcamera 40 and illumination source 105 may be associated with each of theplurality of imaging heads 16 and be used to capture digital cameraimages of various points along the reference edge 15′ of thecorresponding printing plates that is imaged by the specific imaginghead.

In preferred embodiments of the present invention, the digital cameraimages captured by digital camera 40 may be analyzed by one or moreimage data processors (not shown) to identify reference edge 15′ and todetermine the locations of at least two reference points 28A and 28B onreference edge 15′. Controller 20 may include the one or more image dataprocessors. Controller 20 may determine the location of at least tworeference points 28A and 28B and determine the alignment of printingplate 14 relative to imaging drum 12. Controller 20 may provide thenecessary instructions to impart print image 17 onto printing plate 14.When the location of at least two reference points 28A and 28B onreference edge 15′ is determined, print image 17 may be imparted ontoprinting plate 14 in alignment with the determined at least tworeference points 28A and 28B. Controller 20 may include a processor toadjust print image data to produce adjusted print image data that alignsprint image 17 on printing plate 14 relative to at least two referencepoints 28A and 28B. A line detection algorithm may be used to locatereference edge 15′ in each of the captured digital camera images. Abest-fit straight line may be fitted to the located reference edge 15′.The positions of the two or more reference points 28A, 28B on referenceedge 15′ may be determined from the fitted line. Referring back to FIG.3, controller 20 may determine the necessary transformation inaccordance with the determined positions of reference points 28A and 28Bin block 108. The transformation is applied to print image data in block110 to produce transformed print image data. The transformed print imagedata is then communicated to imaging head 16 to impart print image 17 inthe desired alignment with reference edge 15′.

To determine the alignment of printing plate 14 relative to imaging drum12 as well as drum transformation for print image data, the one or moreimage data processors requires positional information (preferably withrespect to imaging drum 12) of the captured camera data of the referencepoints 28A and 28B. Digital camera 40 may be attached to an axiallytranslating point such as carriage 101 or imaging head 16. In eithercase, circumferential or main-scan information of the captured digitalcamera image at a given reference point may be obtained from dataprovided by the imaging drum 12 rotational position encoder (not shown).The encoder defines specific main-scan positions of imaging drum 12 thatare typically indexed to an index start point. The index start point maybe defined by at least one of the registration pins 18A, 18B and 18C ofdrum 12. The encoder provides imaging drum rotational positioninformation that is used to coordinate the activation of imaging head 16as it translates along sub-scan direction to correctly impart an imageonto printing plate 14. Main-scan positional calibration of digitalcamera 40 may include capturing digital camera images of a featureincorporated in the surface of imaging drum 12; the main-scan positionalcoordinates of the feature being known.

The sub-scan positional coordinates of the captured images at thereference points must also be known, since digital camera 40 is mountedon carriage 101 or imaging head 16. Carriage 101 typically moves axiallyin synchronism with the rotation of drum 12. Positional control ofcarriage 101 may be accomplished by numerous methods known in the art.Sub-scan positional calibration of digital camera 40 may be accomplishedby several methods. One method may include capturing digital cameraimages of a feature incorporated in the surface of imaging drum 12; thesub-scan positional coordinates of the feature being known. Anothermethod may include additionally detecting a specific reference point onreference edge 15′ by another means such as a focus laser that is partof imaging head 16. The sub-scan position detected by digital camera 40is then compared to the corresponding coordinates determined by thefocus laser. Yet another method may include imparting an image featureonto printing plate 14 with imaging head 16. Carriage 101 may bepositioned to a specific sub-scan position to capture a digital cameraimage of the feature.

Digital camera pixel scaling calibration determines the number ofmicrons per camera pixel. Digital camera pixel scaling calibration maybe determined by imaging a feature of known size and assessing the howmany pixels wide it is. Yet another method of pixel scaling calibrationmay include imaging a feature onto printing plate 14 at a first knownsub-scan position. Carriage 101 may then be moved to a second knownsub-scan position to image the feature again. Digital camera 40 may beused to capture a digital camera image of the two imaged features, thedistance between the two imaged features being the same as the distancebetween the first and second known sub-scan positions.

In preferred embodiments of the present invention, imaging head 16 anddigital camera 40 are moved axially in the sub-scan direction indicatedby arrow 24, while imaging drum 12 is kept stationary. Digital camera 40may capture digital camera images at sub-scan positions corresponding tothe at least two reference points 28A and 28B. Digital camera 40 maysend data corresponding to each of the digital camera images to an imagedata processor which identifies a representation of the at least a partof reference edge 15′ within the images. The main-scan coordinates ofthe at least two reference points 28A and 28B are determined inaccordance with data provided by the drum rotational encoder (not shown)and the representation of the at least a part of reference edge 15′ atthe at least two reference points 28A and 28B.

The Haar transform is an established mathematical technique in imageprocessing. In a preferred embodiment of the present invention, the Haartransform is used to “pattern match” a prototype edge with the sequenceof values derived from vertically integrating the digital camera imagepixels. The Haar transform is applied to a (narrower) sequence ofvertically integrated prototype edge values to produce a first vector.The Haar transform is also applied to a portion of a sequence of thedigital camera image vertically integrated values to produce a secondvector. The dot product of these two vectors is referred to ascorrelation. Correlation is a measure of the pattern match between theprototype edge and an edge found at that location in the digital cameraimage. This process is repeated for alternate portions of the sequenceof the digital camera image integrated values, to produce a correlationgraph. Each of the alternate portions typically starts at eachconsecutive pixel location of the digital camera image. The location ofmaximum correlation (i.e. the global maximum) has a high probability ofcorresponding to the reference edge portion in the image.

The global maximum of the correlation graph may in some cases, lead toan erroneous result. There may be other local maxima in the graph, oneof which may correspond to the reference edge. A local maximum may belocated by applying a similar wavelet transform to the correlationgraph. A coiflet transform operation may be applied to the entirecorrelation graph, producing a coiflet transform vector. A threshold maybe selected wherein values below the threshold are reduced to zero. Thetransform operation may then be reversed and a modified version of thecorrelation graph reproduced. This technique may be commonly employed inimage compression. In the present invention, the compression applied maybe of a magnitude that the modified version of the correlation graph isa sequential series of width and height scaled coiflet mother wavelets.Each of the local maxima present in the original correlation graph willtypically become the center (peak) of one of the mother wavelets.Finding the locations of the local maxima is simply a matter of listingthe locations of the mother wavelets. In this way, an image may haveseveral possible choices of locations for the imaged portion of thereference edge, some more likely to be correct than others.

It is to be understood that the present invention is not limited to theuse of the Haar transform and suitable correlation or convolutionalgorithm may be used to distinguish between the prototype edge andvideo images. In addition, speed improvements may be made by settingHaar transform vector values to zero if they are under a predeterminedthreshold before taking the dot product. The present invention mayfurther use any suitable image processing method and associated edgedetection algorithm to distinguish the portion of reference edge 15′captured in the video frames. The position of the least two referencepoints 28A and 28B may be determined by the identification of theselocations and from imaging drum 12 and carriage 101 positionalinformation during the capturing of the images at reference points 28Aand 28B. The determined locations of the at least two reference pointsreference 28A and 28B may then be used to determine a transform to applyto print image data such that when the transformed print image data iscommunicated to imaging head 16 and its associated radiation source,print image 17 is substantially aligned with reference edge 15′.

Printing plate 16 and imaging drum 12 may have surface imperfectionsthat may appear to produce images that may obscure the contrast of thereference edge 15′ at the detected positions. The surface imperfectionsthemselves may have a form and shape that may lead to erroneous resultsif the edge detection algorithms employed mistakenly interpret theimperfections as part of reference edge 15′ Erroneous results may alsooccur if the edge detection algorithms interpret regular imaging drum 12features as part of reference edge 15′. A plurality of locationsoriented along the sub-scan direction 26 may be imaged by digital camera40 and defined by a suitably chosen edge detect algorithm. The pluralityof locations may be greater in number than the at least two referencepoints 28A and 28B. If each location produces at least one edge value, abest-fit straight line may then be fitted through these points. Thebest-fit straight line forms a relationship between the determinedsub-scan or axial locations of the plurality of points and theircorresponding circumferential locations to assess the accuracy of thedetermined locations with respect to the straight line thattheoretically represents a straight plate edge.

Each digital camera image from the plurality of locations along thesub-scan direction may instead result in a plurality of possiblereference edge positions in at least one of the locations, eachassociated with a figure of merit. An algorithm for fitting a straightline can be designed to select from the possible reference edgelocations, with a higher weighting for edge locations with a high figureof merit. If one or a few of the high figure of merit reference edgelocations do not lie in a straight line and a lower figure of merit edgelocation does lie nearer the straight line, it may be selected instead.Standard methods for best straight-line fitting may be applied to theselected set of reference edge locations. The locations of referencepoints 28A and 28B will typically lie on, or very close to the fittedstraight line. Once the locations of the at least two reference points28A and 28B are confirmed and/or adjusted, the transformation for printimage data may be determined.

Certain implementations of the invention comprise computer processorsthat execute software instructions that cause the processors to performa method of the invention. For example, one or more data processors incontroller 20 may implement method 100 of FIG. 3 by executing softwareinstructions in a program memory accessible to the processors. Theinvention may also be provided in the form of a program product. Theprogram product may comprise any medium which carries a set ofcomputer-readable signals comprising instructions which, when executedby a computer processor, cause the data processor to execute a method ofthe invention. Program products according to the invention may be in anyof a wide variety of forms. The program product may comprise, forexample, physical media such as magnetic data storage media includingfloppy diskettes, hard disk drives, optical data storage media includingCD ROMs, DVDs, electronic data storage media including ROMs, flash RAM,or the like or transmission-type media such as digital or analogcommunication links.

The backlit edge method and apparatus of the present invention aredescribed at the hand of FIGS. 5, 6 and 7. FIG. 6 is a cross-section ofdrum 12 and of clamp 130 located at point 28B of FIG. 5. In FIG. 6printing plate 14 is held to surface 13 of drum 12 by clamp 130 suchthat reference edge 15′ of printing plate 14 protrudes over drum slot140 in drum 12. As is shown in FIG. 5, carriage 101 may be moved suchthat digital camera 40 is in a position to image reference edge 15′ ofprinting plate 14 at point 28B through clamp 130 and that illuminationsource 105 may simultaneously illuminate reference edge 15′ at point 28Bthrough clamp 130. This arrangement is shown in detail in FIG. 6, inwhich illumination source 105 illuminates diffusely reflective layer 150that has a diffusely reflective surface, located in the bottom of drumslot 140 on a radially recessed surface, through illumination baffle 170of clamp 130 with illuminating light beam 160. Diffusely reflected light180 is gathered by digital camera 40 through imaging aperture 190 inclamp 130 and is used by digital camera 40 to capture an image of point28B. The illuminating is therefore performed on the side of the printingplate that is in contact with the imaging drum. In FIG. 6 reference edge15′ is specifically shown with a bevel angle produced by the cutting ofthe un-imaged printing plate 14. If printing plate 14 and reference edge15′ were to be illuminated from the top instead of as in the presentinvention, the light reflected from the top surface of printing plate 14and the light reflected from the bevel edge of reference edge 15′ wouldmake it very difficult to determine the actual mechanical edge ofreference edge 15′ of printing plate 14. By diffusely illuminating thesurface of printing plate 14 that faces away from digital camera 40,using the light diffusely reflected by diffusely reflective layer 150,the contrast between the true mechanical edge of reference edge 15′ anddiffusely reflective layer 150 is much improved through the reflectionof light from any surfaces of printing plate 14 having been limited.This allows more accurate determination of the true mechanical edge ofreference edge 15′ of printing plate 14 by the image analysis methodsdescribed herein.

In a further embodiment of the present invention, the contrast may befurther enhanced, and the true mechanical edge of reference edge 15′ ofprinting plate 14 more precisely determined, by employing thearrangement of FIG. 7. FIG. 7 shows a cutaway of drum slot 140 in drum12 of FIG. 5. Printing plate 14 having beveled reference edge 15′ withmechanical edge 200 is clamped to the cylindrical surface 13 of drum 12by a clamp (not shown in the interest of clarity). Mechanical edge 200has perpendicular projection 240 on diffusely reflective layer 150 givenby line a-a′. In this embodiment of the present invention diffuselyreflective layer 150 has upon its surface facing digital camera 40 aplurality of non-reflective areas 210 a, 210 b and 210 c. Any shape maybe chosen for the non-reflective areas 210 a, 210 b, 210 c, thoughshapes having perimeters that form at least one acute angle withreference edge 15′ are preferred. In FIG. 7 non-reflective areas 210 a,210 b, 210 c, in the form of diagonally slanted strips, have been chosenas being one simple choice that satisfies this preference. Acutereflective apex 230 is formed in the reflective part of diffuselyreflective layer 150 between perpendicular projection 240 andnon-reflective area 210 a. Similar acute reflective apexes are formedbetween perpendicular projection 240 and non-reflective areas 210 a and210 b and are not indicated in FIG. 7 for the sake of clarity. The imageof point 28B obtained by digital camera 40 comprises at least onenon-reflective area 210 a, at least one acute reflective apex 230 andreference edge 15′. The image of reference edge 15′ so obtainedcomprises mechanical edge 200, if a bevel is present on the particularprinting plate 14. The illuminating of reference edge 15 is thusspatially interrupted along an interrupting section of that part of thereference edge 15′ that is associated with point 28B.

Given that light diffusely reflected from diffusely reflective layer 150may potentially illuminate the beveled surface of printing plate 14along reference edge 15′, non-reflective areas 210 a, 210 b, 210 c,provide for regions of mechanical edge 200 of reference edge 15′,corresponding to non-reflective areas 210 a, 210 b, 210 c, substantiallynot being illuminated at all. On the other hand, regions of mechanicaledge 200 of reference edge 15′, corresponding to reflecting region 220of diffusely reflective layer 150 may conversely be illuminated,depending on the angle of the bevel of reference edge 15′. By imagingreference edge 15′ in the vicinity of acute reflective apex 230mechanical edge 200 of reference edge 15′ may be determined veryaccurately in the illuminated area adjacent to acute reflective apex230. In regions of mechanical edge 200 of reference edge 15′, protrudingover non-reflective areas 210 a, 210 b, 210 c, mechanical edge 200cannot be identified for lack of illumination, while, in regions ofmechanical edge 200 of reference edge 15′ protruding over reflectingregion 220 of diffusely reflective layer 150, illumination of thebeveled surface of reference edge 15′ by stray diffusely reflectedlighted light from reflecting region 220 may still potentially inducesmall errors in the locating of mechanical edge 200 in the image.Optimally accurate determination of the location of mechanical edge 200therefore occurs in those regions of reference edge 15′ protruding overacute reflective apex 230 of the reflective part of diffusely reflectivelayer 150. Again, the determination of mechanical edge 200 from theimage obtained by camera 40 at point 28B occurs by the analysis processalready described. It is to be noted that, in the case of a printingplate 14 having reference edge 15′ with a bevel of the opposite sense tothat shown in FIGS. 6 and 7, mechanical edge 200 is the outer edgeimaged by default by digital camera 40 and no light directly reflectedby that beveled surface can reach digital camera 40 to create an imagethat might mislead the user as to the exact location of mechanical edge200.

Since reference edge 15′ has to be determined at least two axial points28A and 28B along drum 12 in order to determine the required imagerotation, the arrangement described here may be repeated at a pluralityof points along the clamping system of drum 12. Typical drum systemshave continuous or segmented clamp arrangements, spanning substantiallythe entire axial width of drum 12. In a further implementation of thepresent invention a single clamp 120, 130 can therefore have a pluralityof mutually fixed arrangements of illumination baffles 170 and imagingapertures 190, the result being that, in any chosen region along theaxial length of reference edge 15′ there is always a nearby set ofillumination baffle 170 and imaging aperture 190 that can be used toimplement the edge detection method of the present invention.

In yet a further embodiment of the present invention, a series ofnon-reflective areas 210 a, 210 b, 210 c are fashioned on diffuselyreflective layer 150 in the vicinity of a chosen point 28B such that theimage captured by digital camera 40 comprises a plurality of images ofnon-reflective areas 210 a, 210 b, 210 c. This provides a plurality ofacute reflective apexes 230 at which mechanical edge 200 can bedetermined, thereby improving the accuracy of the analysis yet further.Non-reflective areas may be fashioned on diffusely reflective layer 150along substantially the entire length of slot 140.

In FIG. 8 yet a further embodiment of the present invention shows a planview of drum slot 140 of drum 12 at point 28B of FIG. 5, as illuminatedby illumination source 105 (not shown). Non-reflective areas 250 a, 250b, 250 c, 250 d, 250 e and 250 f on diffusely reflective layer 150 haveedges making very acute angles with perpendicular projection 240 ofmechanical edge 200 of reference edge 15′ of printing plate 14, whichprotrudes over the edge of drum slot 140. In FIG. 8, printing plate 14,reference edge 15′ and mechanical edge 200 are not shown for the sake ofclarity and perpendicular projection 240, denoted by the line a-a′,represents the circumferential location of mechanical edge 200 in theimage of point 28B. Similarly, clamp 130, which clamps printing plate 14to cylindrical surface 13 of drum 12, as in FIG. 5, is not shown in FIG.8 for the sake of clarity. Acute reflective apex 260, in this embodimentof the present invention, is very acute. Any cross-slot repositioning ofreference edge 15′, and thereby of perpendicular projection 240, willcause the position of acute reflective apex 260 to move by a largeamount in the axial direction of drum 12 along perpendicular projection240. To ensure that there is always at least one acute reflective apexin the field of view of digital camera 40, non-reflective areas 250 a,250 b, 250 c are fashioned in high density across slot 140 as shown inthe circumferential direction of drum 12. As a result, a plurality ofnon-reflective areas 250 a, 250 b, 250 c will be crossed byperpendicular projection 240 as reference edge 15′ is repositionedcircumferentially with respect to drum 12 (of FIG. 5) over drum slot140. This results in an increased likelihood of an acute reflective apex260 being located in the image obtained by digital camera 40.Additionally, the fact that acute reflective apex 260 is more acute inthis embodiment of the present invention, allows a larger vicinity ofacute reflective apex 260 to be employed in locating perpendicularprojection 240, and, thereby, mechanical edge 200. This inherentlyincreases the accuracy of the method.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

 10 plate-making machine (platesetter)  11 third reference point onorthogonal edge  11′ registration surface on orthogonal edge  12 drum 13 cylindrical surface  14 printing plate  15 reference edge of plate 15′ reference edge of (skew) plate  16 imaging head  17 print image 17A edge of print image  18A registration pin  18B registration pin 18C registration pin  18A′ registration surface of punching machine 18B′ registration surface of punching machine  19 edge of plateorthogonal to 15  19′ edge of (skew) plate orthogonal to 15′  20controller  21 reference point  22 memory  23 circumferential distance 24 direction parallel to the axis of drum  25 region adjacent toreference edge 15  26 circumferential main-scan direction (forward orbackward)  27 print image data  28A first of at least two referencepoints on reference edge  28B second of at least two reference points onreference edge  40 digital camera  50 punching machine  52 punch table 60 printing press  62 press cylinder 100 registering and impartingprint image onto printing plate 101 carriage 102 mount plate 103 leadscrew 104 locate points on registration edge 105 illumination source 106determine required rotation (θ) 108 determine transformation 110 applytransformation 112 image plate 120 clamp 130 clamp 140 drum slot 150diffusely reflective layer 160 illuminating light beam 170 illuminationbaffle 180 diffusely reflected light 190 imaging aperture 200 mechanicaledge 210a non-reflective area 210b non-reflective area 210cnon-reflective area 220 reflecting region 230 acute reflective apex 240perpendicular projection 250a non-reflective area 250b non-reflectivearea 250c non-reflective area 250d non-reflective area 250enon-reflective area 250f non-reflective area 260 acute reflective apex

1. A method of imparting a print image on a printing plate, the methodcomprising: a) mounting the printing plate on an imaging drum in anorientation wherein a reference edge of the printing plate extends alongthe drum in a substantially axial direction; b) determining a locationof at least one point on the reference edge, wherein the determiningcomprises: i) illuminating a region, the region comprising at least apart of the reference edge associated with each of the at least onepoint; ii) capturing at least one digital camera image of the region;and iii) locating the at least a part of the reference edge in thedigital camera image; c) imparting the print image on the printing platein alignment with at least the determined location of the at least onepoint; wherein the locating at least a part of the reference edgecomprises locating a mechanical edge of the printing plate; wherein theilluminating is performed on the side of the printing plate that is incontact with the imaging drum; wherein the illuminating is by diffusereflected light reflected from at least one diffusely reflective area onthe imaging drum; wherein the illuminating is spatially interrupted byat least one non-reflective surface on the imaging drum; wherein thereference edge is positioned to extend over a slot in the imaging drum;the at least one non-reflective surface and the at least one diffuselyreflective surface are on a radially recessed surface of the slot; andwherein the illuminating is spatially interrupted along an interruptingsection of a part of the reference edge associated with the at least onepoint.
 2. The method as in claim 1, wherein the at least one diffuselyreflective area forms an acute reflective apex with the perpendicularprojection of the reference edge onto the radially recessed surface ofthe slot.
 3. The method as in claim 2, wherein: a) the printing plate isheld to the drum by at least one clamp; b) the capturing of at least onedigital camera image of the region is done through an imaging aperturein the at least one clamp; and c) the illuminating is done through anillumination baffle in the at least one clamp.
 4. A method as in claim1, wherein the locating the at least part of the reference edgecomprises applying a correlation algorithm to distinguish an image ofthe at least a part of the reference edge from a prototype edge.
 5. Amethod as in claim 4, wherein the applying the correlation algorithmcomprises applying a Haar transform.
 6. A method as in claim 4, whereinthe applying the correlation algorithm comprises applying a coiflettransform.
 7. A method of determining an alignment of a printing platerelative to an imaging drum on which the printing plate is mounted, themethod comprising: a) determining a location of at least one point on areference edge of the printing plate, wherein the reference edge extendsalong the imaging drum in a substantially axial direction; thedetermining a location comprising: i) illuminating a region, the regioncomprising at least a part of the reference edge associated with the atleast one point; ii) capturing at least one digital camera image of theregion with a digital camera; and iii) locating the at least a part ofthe reference edge in the digital camera image; b) determining thealignment of the printing plate at least in part from: i) the locationof the at least a part of the reference edge in the digital camera imageof the at least one point; and ii) a position of the digital camerarelative to the imaging drum during the capturing of the at least onedigital camera image of the at least one point; wherein the locating atleast a part of the reference edge comprises locating a mechanical edgeof the printing plate; wherein the illuminating is performed on the sideof the printing plate that is in contact with the imaging drum; whereinthe illuminating is by diffuse reflected light reflected from at leastone diffusely reflective area on the imaging drum; wherein theilluminating is spatially interrupted by at least one non-reflectivesurface on the imaging drum; wherein: a) the reference edge ispositioned to extend over a slot in the imaging drum; b) the at leastone non-reflective surface and the at least one diffusely reflectivesurface are on a radially recessed surface of the slot; and wherein theilluminating is spatially interrupted along an interrupting section of apart of the reference edge associated with the at least one point. 8.The method as in claim 7, wherein the at least one diffusely reflectivearea forms an acute reflective apex with the perpendicular projection ofthe reference edge onto the radially recessed surface of the slot. 9.The method as in claim 8, wherein: a) the printing plate is held to thedrum by at least one clamp; b) the imaging of the reference edge is donethrough an imaging aperture in the at least one clamp; and c) theilluminating is done through an illumination baffle in the at least oneclamp.
 10. A method as in claim 7, wherein the at least one pointcomprises at least two axially spaced apart points on the referenceedge.
 11. A method as in claim 7, wherein the digital camera is mountedon a carriage operable for moving in an axial direction relative to theimaging drum, the method further comprising determining the alignment ofthe printing plate from an axial position of the carriage.
 12. A methodfor determining the location of a reference edge of a plate relative toa substrate on which the plate is mounted, the method comprisingdetermining a location of at least one point on the reference edge,wherein the reference edge extends along a first direction on thesubstrate; the determining a location comprising: a) illuminating aregion, the region comprising at least a part of the reference edgeassociated with the at least one point; b) capturing at least onedigital camera image of the region with a digital camera; c) locatingthe at least a part of the edge in the digital camera image; d)determining the location of the at least one point from: i) a part ofthe reference edge in the digital camera image of the at least onepoint; ii) a position of the digital camera relative to the substrateduring the capturing of the at least one digital camera image of the atleast one point; wherein the locating at least a part of the referenceedge comprises locating a mechanical edge of the printing plate; whereinthe illuminating is performed on the side of the plate that is incontact with the substrate; wherein the illuminating is by diffusereflected light reflected from at least one diffusely reflective area onthe substrate; wherein the illuminating is spatially interrupted by atleast one non-reflective surface on the substrate; and wherein: a) thereference edge is positioned to extend over a slot in the substrate, theslot extending in substantially the first direction; b) the at least onenon-reflective surface and the at least one diffusely reflective surfaceare on a recessed surface of the slot; and wherein the illuminating isspatially interrupted along an interrupting section of a part of thereference edge associated with the at least one point.
 13. The method asin claim 12, wherein the at least one diffusely reflective area forms anacute reflective apex with the perpendicular projection of the referenceedge onto the recessed surface of the slot.
 14. The method as in claim12, wherein: a) the plate is held to the substrate by at least oneclamp; b) the imaging of the reference edge is done through an imagingaperture in the at least one clamp; and c) the illuminating is donethrough an illumination baffle in the at least one clamp.
 15. Anapparatus as in claim 14, wherein the substrate is an imaging drum. 16.A method as in claim 12, wherein the at least one point comprises atleast two spaced apart points on the reference edge.
 17. A method as inclaim 12, wherein the digital camera is mounted on a carriage operablefor moving in the first direction relative to the surface, the methodfurther comprising determining the location of the reference edge from aposition of the carriage.